1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and an image forming apparatus provided with this optical scanning apparatus. In particular, the invention relates to an optical beam scanning apparatus which in an overillumination scanning optical system in which a width of an luminous flux made incident on a polygon mirror is wider than a width of one reflecting surface forming the polygon mirror, is able to scan the luminous flux on a photoconductive drum and an image forming apparatus provided with this optical beam scanning apparatus.
2. Related Art
In recent years, in image forming apparatus of an electrophotographic mode, for example, laser printers, digital copiers and laser facsimiles, an optical beam scanning apparatus for irradiating laser light (optical beam) on a surface of a photoconductive drum and scanning the laser light to form an electrostatic latent image on the photoconductive drum is provided.
Recently, in order to devise to realize high-speed scanning on a surface of a photoconductive drum, for example, a method in which plural light sources (laser diodes) are provided in one laser unit, thereby increasing the number of laser light (multibeam mode) is proposed. In this multibeam method, plural beams for every color component emitted from each of light sources (for example, yellow, magenta, cyan, and black) are processed in a pre-deflection optical system and converted into one beam, which is then made incident on a polygon mirror. The beam deflected by the polygon mirror is mediated through an fθ lens configuring a post-deflection optical system and subsequently separated into a beam for every color component and irradiated on a photoconductive drum of every color component.
Here, the rotation axis direction of the polygon mirror as a deflector is defined as “sub-scanning direction”, and a direction vertical to each of the optical axis direction of the optical system and the rotation axis direction of the deflector (polygonal mirror) is defined as “main scanning direction”. Incidentally, the sub-scanning direction in the optical system is corresponding to a conveyance direction of a transfer material in an image forming apparatus, and the main scanning direction in the optical system is corresponding to a direction vertical to the conveyance direction within a surface of the transfer material in the image forming apparatus. Also, an image surface shows the surface of the photoconductive drum, and an imaging surface shows a surface on which a luminous flux (laser light) actually forms an image.
In general, a relation expressed by [Expression 1] is present among an image processing rate (paper conveyance rate), an image resolution, a motor rotation rate and a number of polygon mirror surfaces.
                              P          *          R                =                              25.4            *            Vr            *            N                    60                                    [                  Expression          ⁢                                          ⁢          1                ]            In the foregoing expression, P (mm/s) represents a processing rate (paper conveyance rate); and R (dpi) represents an image resolution (number of dots per inch) Also, Vr (rpm) represents a number of revolutions of polygon motor; and N represents a number of polygon mirror surfaces.
As expressed by the foregoing [Expression 1], the printing speed and resolution in the image forming apparatus are proportional to the number of revolutions of polygon motor (Vr) and the number of polygon mirror surfaces (N). Accordingly, in order to realize high resolution as well as high speed in the image forming apparatus, it is necessary to increase the number of polygon mirror surfaces (N) or to raise the number of revolutions of polygon motor (Vr).
However, in a conventional general underillumination scanning optical system, a width of a luminous flux (laser light) made incident on a polygon mirror in a main scanning direction is made smaller than a width of one reflecting surface forming the polygon mirror in the main scanning direction (reflection width), thereby reflecting the whole of the luminous flux (laser light) made incident on the polygon mirror.
However, since not only a beam diameter on the image surface is proportional to an F number, but also the F number is expressed by Fn=f/D wherein f represents a focal distance of the imaging optical system, and D represents a beam diameter of the main scanning direction on the polygon mirror surface, when it is intended to make the beam diameter on the image surface small for the purpose of devising to realize high image quality, the beam diameter of the main scanning direction on the polygon mirror surface must be made large.
In other words, in order to obtain high image quality at a certain fixed level or more, there is present a restriction that the beam diameter of the main scanning direction on the polygon mirror surface must be regulated to a fixed size or more.
Nevertheless, in order to realize high resolution as well as high speed, when it is intended to increase the number of polygon mirror surfaces (N), the polygon mirror itself must be increased in size. As a result, when it is intended to rotate a large-sized polygon mirror at a high speed, a load to a motor for driving the polygon mirror becomes large, and the motor cost increases. In addition, at the same time, the noise or vibration of the motor or the generation of a heat becomes large, and a countermeasure thereto becomes necessary separately.
Then, an image forming apparatus using an over-illumination scanning optical system is proposed in place of the underillumination scanning optical system. In the overillumination scanning optical system, a width of a luminous flux made incident on a polygon mirror is made wider than a width of one reflecting surface forming the polygon mirror.
According to this, it is possible to reflect the luminous flux by using the entire surface of the reflecting surface forming the polygon mirror (or plural reflecting surfaces); and even in the case where it is intended to ensure the beam diameter on the polygon mirror surface while increasing the number of reflecting surfaces of polygon mirror (N) for the purpose of devising to realize high resolution as well as high speed, it is possible to make the diameter of the polygon mirror itself small. Accordingly, a load to a motor for driving the polygon mirror can be reduced, and the motor cost can be reduced. Also, since not only the diameter of the polygon mirror itself can be made small, but also the number of reflecting surfaces can be increased, it is possible to make the shape of the polygon mirror close to a circle, and it is possible to make the air resistance at the time of driving the polygon mirror low. As a result, even when the polygon mirror is rotated in a high speed, it is possible to reduce the noise or vibration and the generation of a heat.
Furthermore, following the reduction in the noise or vibration and the generation of heat, the whole or a part of countermeasures parts for reducing the noise or vibration, such as glasses, become unnecessary, and the costs in manufacturing an image forming apparatus can be lowered. Also, a high duty cycle becomes possible.
The foregoing overillumination scanning optical system is described in, for example, Leo Beiser, Laser Scanning Notebook, SPIE OPTICAL ENGINEERING PRESS.
In general, in scanning a luminous flux made incident from a semiconductor laser device on a photoconductive drum, in the underillumination scanning optical system, an edge part of a polygon mirror is not used, whereas in the overillumination scanning optical system, an edge part of a polygon mirror is used.
For that reason, since an error in the shape in the edge part of the main scanning direction of the polygon mirror is large, when a luminous flux is deflected (scanned) by using the edge part, the luminous flux to be deflected (scanned) becomes dull, whereby a wave front aberration is deteriorated in the entire scanning region on the photoconductive drum. As a result, the beam diameter on the photoconductive drum as an image surface increases, whereby a beam profile is deteriorated.